Imitate Copy Transform Constraint in BGE
as I understood this script may help you.

from bge import logic

cnt = logic.getCurrentController()
scn = logic.getSceneList()[0] # the background scene or low-poly scene
cam1 = cnt.owner # the high-poly scene camera
cam2 = scn.objects['Low_poly_camera'] # the low-poly scene camera

def c2tc1(): # we set the cam2 pos/rot to the cam1
    cam2.worldPosition = cam1.worldPosition
    cam2.worldOrientation = cam1.worldOrientation

c2tc1()


How it works:

this code should run always with high-poly scene camera that moves, you don't need to move the low-poly scene camera it will follow the high camera always.

Make sure that:

True level triggering  pulse mode is active so the code should run on each frame.